Spinels, in general are a class of minerals having the general formula A2+B23+O4− Aluminum spinels include natural spinels, MgAl2O4, gahnite, ZnAl2O4, hercynite, and FeAl2O4. Although spinels are found in nature, due to increasing demand synthetic spinels are now being made by various methods.
Presently, spinels are used as windows and domes in high-speed missiles and pods. Although silicate glasses are suitable for most common window uses, they are too weak to be used in the latter applications. A magnesium aluminate spinel is a polycrystalline ceramic material that has excellent optical and mechanical properties.
The synthesis of magnesium aluminate spinels is well-known in the literature. The common methods of synthesis can be divided into three basic classes: co-precipitation of aluminum and magnesium salts; ball milling of magnesium and aluminum compounds; and wet mixing of alumina and magnesium compounds. Each of these processes has inherent limitations which may limit their utility on an industrial or commercial scale.
Generally, the methods for producing magnesium aluminate spinels involve contacting an alumina compound with a magnesium compound under specific process conditions to obtain a slurry or solution that is then dried and calcined to form the spinel.
Such methods are described in a number of patents. U.S. Pat. No. 4,400,431 describes a method for preparing polymetallic spinels by co-precipitation of the metals in an alkali solution having a pH in the range of 8 to 10. EP 0110704 describes a process of adding an alumina component to a liquid phase having a pH of 8.5 to 10.5, preferably 9 to 10. WO 86/06090 describes a method of preparing spinel compositions by combining an acidic solution containing at least one alkaline earth metal component with a basic solution containing at least one alumina component to form a combined mass including a liquid phase and an alkaline earth metal, Al-containing precipitate. This application specifically requires that the pH of the liquid phase during the combination be maintained at a pH of 7 to 9.5, preferably 7 to 8.5.
U.S. Pat. No. 6,517,795 describes a process for producing hydrotalcites, and metal oxides thereof, by calcination. The hydrotalcites are obtained by hydrolyzing mixtures of metal alcoholates. As per this patent the pH value of the water for hydrolysis may be in the range of 0 to 14, preferably 1 to 13. A preferred pH range is not taught.
Akzo Nobel NV has also patented processes in this area of technology for example in patent applications WO 99/41195, and WO 2000/44671. In these applications, it is described that an aluminium source and a magnesium source are fed into a reactor and reacted in an aqueous suspension to obtain an anionic clay. The invention describes that the pH of this reaction is to be controlled e.g. by the use of a pH modifier ammonium base. The modifier may be added to the slurry before or during the reaction or to the individual reactants before their combination in a slurry. The preferred pH ranges are not described in the applications however, examples in the respective patents use a pH between 6 and 10.
WO 2013/155518 describes a process for the production of magnesium aluminate spinels in which boehmite alumina is hydrothermally aged in the presence of a magnesium compound, or in which the boehmite alumina is aged hydrothermally (apart) and then contacted with a magnesium compound. Either material can be dried and calcined to produce a spinel. Again, no preferred pH range is mentioned.
As seen from the above, the prior art teaches that a basic pH is required when adding the alumina to an aqueous solution.
The inventors of the present invention have found that the prior art methods produce magnesium aluminate spinels with significantly varying properties e.g. the surface area, etc. of the magnesium aluminate spinels. The need has been identified to control these properties, especially the surface area, to produce spinels of improved quality which can be used in a variety of applications.